ICTS

In low dimensional superconductors, disorder and many body interactions leads to a veritable zoo of intriguing phases and phase transitions. A exemplar of the same is for instance the highly debated anomalous metallic phase which exists circumventing the celebrated Anderson localisation. On the other hand, disorder itself triggers a host of very intriguing and exotic effects such as the Quantum Griffiths Phase (QGP) that stems from the associated Infinite Randomness Fixed Point (IRFP). In this talk we provide incontrovertible proof of emergence of a QGP in the 2D electron gas formed in the LaScO_3 /SrTiO_2 heterostructure. The signatures of the QGP are embedded in the magneto-resistance exhibited by the sample. In particular, we show that in the Griffiths phase (that obtains at higher temperatures), the effective dynamical exponent diverges as a function of the magnetic field. Further, at lower temperatures we argue that the divergence signalling the QGP is cut-off by interplay of disorder and long range interactions. We present plausible mechanisms that can explain this phenomenon of the emergence of the QGP and also its eventual demise. We also show that the killing-off of the QGP also leads to the destruction of the quantum critical point between the superconductor and the normal state.

In this talk, I will discuss the charge correlations in two different classes of kagome metals, each with electron fillings near saddle points in their band structures. In the first compound, CsV3Sb5, a dominant breathing mode of the kagome network drives the formation of a metastable charge density state. Charge correlations in this state undergo an unusual evolution upon tuning the carrier filling, suggesting the presence of a nearby nematic instability. In the second compound, ScV6Sn6, charge order is driven by an out-of-plane instability of the Sc-Sn chains that thread through the kagome planes. This drives a form of frustrated charge order likely responsible for the pseudogap and anomalous electronic properties reported in this material. The differing routes to charge order across multiple families of kagome metals will be discussed.

Coulomb interactions among charge carriers have a profound impact on the macroscopic properties of materials. At sufficient strength, these interactions can give rise to captivating phenomena such as quantum criticality, Mott-Hubbard states, and unconventional superconductivity. Consequently, the search for new families of strongly correlated materials hosting a diverse range of quantum phases is a central research theme in condensed matter physics. In this work, we present experimental evidence obtained from scanning tunneling microscopy measurements for a cascade of strongly correlated states appearing in the partially occupied kagome flat bands of Co1−xFexSn with finite Fe doping x. Unlike in conventional strongly correlated materials, the kagome flat bands arise from a quantum interference effect imparted by geometric frustration [1]. At elevated temperatures (T ≥ 16 K), we observe that strong local Coulomb interactions (U > 100 meV) blend the states of two kagome flat bands across a broad doping range, resulting in an inter-band state that exhibits a nematic order parameter. This strongly coupled state serves as the parent phase of a Mott-Hubbard state, which arises in samples with ideal Fe doping (x = 0.17) and descends into charge ordered states upon doping with both electrons and holes [2]. These observations suggest a significant degree of electronic interactions within the partially filled kagome flat bands over a wide doping range, driven by the combination of strong Coulomb repulsion and the orbital degeneracy of the Co atoms. Our research expands the realm of Mott-Hubbard states to unconventional flat bands and introduces a new avenue for investigating strongly correlated quantum phases of matter.
We gratefully acknowledge support by the Hong Kong RGC and the Croucher Foundation.
[1] C. Chen et al., Phys. Rev. Research 5, 043269 (2023)
[2] C. Chen et al., submitted (2024)

Kagome lattices are known for their outstanding lattice-driven band structure properties, which may give rise to a plethora of different physical properties, such as electronic correlations, charge and spin density waves, superconductivity, and topological states of matter [1]. Due to its Star-of-David atomic arrangement, it is expected to obtain saddle points, flat bands, and Dirac cones in its band structure independent of the details of the compounds [1]. Although some reports claim the presence of Dirac points and saddle points close to the Fermi level, it is still a rare occurrence to obtain a kagome lattice with a flat band located at the Fermi level [2-5]. Interestingly, in this condi-tion, it would be possible to unravel quantum criticality, which raises questions about correlations, topology, and unconventional superconductivity [6]. In this work, we locally explore a potential candidate with flat bands near the Fermi level through scanning tunneling microscopy/spectroscopy (STM/S). We are able to directly make our measurements in the kagome termination, which brings important insights to our STS. Our point STS shows clear many-body phenomena signatures, which are corroborated by temperature and magnetic field dependencies measurements. Additionally, through spectroscopic imaging STS and a direct comparison to ab initio band structure calculations, we can identify the dominating scattering bands near the Fermi level. Finally, we constructed a supercell [7] from our spectroscopic imaging STS, unraveling the unique positioning of the flat band at the kagome layer. In the end, we discuss the possible implications of flat bands and the appearance of a correlated metal phase.

1. J.-X. Yin, B. Lian, and M. Z. Hasan, Nature 612, 647-657 (2022).

2. L. Ye et al., Nature 555, 638-642 (2018).

3. Y. Hu et al., Nat. Commun. 13, 2220 (2022).

4. L. Ye, Nat. Phys. 20, 610-614 (2024).

5. Y. Guo et al., arXiv 2406.05293 (2024).

6. L. Chen et al., arXiv 2307.09431 (2023).

7. I. Zeljkovic et al., Nat. Mater. 11, 585-589 (2012).

Electronic flat band systems have gained considerable attention as a platform to realize quantum matter phenomena including the fractional quantum Hall effect, topological phases, unconventional superconductivity, and new forms of magnetism. Electrons in a flat band are characterized by a quenched kinetic energy (bandwidth), a diverging effective mass, and localized wave functions, leading to strong correlation physics in the presence of Coulomb interactions. In recent years, significant efforts have been directed at realizing electronic flat bands in special ‘line graph’ lattices, which under certain conditions exhibit eigenstates that are spatially ‘trapped’ by the total destructive interference of hopping pathways (compact localized states). The realization of new flat band systems and their tuning via chemical composition, carrier doping, or lattice geometry (dimensionality) is an exciting frontier for engineering quantum phases of matter at potentially high temperatures.
In this talk, I will discuss how flat bands are not just a textbook realization of the line-graph band structure toy model in the noninteracting limit, but rather can support strong correlation physics in two paradigmatic lattices – the kagome and pyrochlore lattice. We investigated the electronic band structure in kagome and pyrochlore metals using angle-resolved photoemission spectroscopy (ARPES). I will first discuss the general phenomenology of flat bands in a few representative kagome and pyrochlore intermetallics [1,2]. Next, I will present evidence for one-dimensional flat bands giving rise to heavy fermion-like behavior in kagome metal Ni3In [3]. Last, I will show recent work elucidating the band structure of ‘f electron-free’ heavy-fermion compound LiV2O4, and the role of flat bands in this system [4].

[1] M. Kang, et al., Topological flat bands in frustrated kagome lattice CoSn. Nature Comm. 11, 4004 (2020).
[2] J. Wakefield, et al., Three-dimensional flat bands in pyrochlore metal CaNi2, Nature 623, 301 (2023).
[3] L. Ye, et al., A flat band-induced correlated kagome metal. Nature Phys. 20, 610 (2020).
[4] D. Oh, et al., in preparation.

The phase diagram of the kagome metal family AV3Sb5 (A = K, Rb, Cs) features both super- conductivity and charge density wave (CDW) instabilities, which have generated tremendous attention. We've studied the temperature evolution and demise of the CDW state using low resolution X-ray diffraction x-ray scattering study of CsV3Sb5 over a broad range of temperatures from 300 K to ∼ 2 K, below the onset of its super-conductivity at Tc ∼ 2.9 K. Such measurements are optimized for diffuse X-ray scattering. Order parameter measurements of the 2 × 2 × 2 CDW structure show an unusual and extended linear temperature dependence onsetting at T ∗ ∼ 160 K, much higher than the susceptibility anomaly associated with CDW order at TCDW = 94 K. This implies strong CDW fluctuations exist to ∼ 1.7 × TCDW. The CDW order parameter is also observed to be constant from T = 16 K to 2 K, implying that the CDW and superconducting order co-exist below Tc, and, at ambient pressure, any possible competition between the two order parameters is manifested at temperatures well below Tc, if at all. These low resolution X-ray measurements will be put into the context of other studies of diffuse scattering and long range order in quantum materials.

Competing many-body phases and chiral superconductivity on the triangular lattice

Two dimensional di-chalcogenide materials with Ising spin orbit coupling often show enhanced charge density wave transition temperature and signatures of unconventional superconductivity unlike their three dimensional bulk counterparts. Some of these experiments in monolayer NbSe2 include enhancement of superconducting transition temperature with increasing disorder concentration, observation of two fold symmetric gap in the presence of in plane magnetic field, and observation of Leggett modes in tunnelling experiments.
In this talk, we will first present our calculation of 3Q CDW ordering and role of disorder in normal state of monolayer NbSe2. We will then present our results for the superconducting state assuming a spin fluctuation mediated pairing scenario and explore the dominant gap functions within this theory. Finally, we will discuss the effects of an in-plane Zeeman field on the homogenous superconducting state.
Indian Institute Of Technology Ma